1. Field of the Invention
The present invention generally relates to a device for generating an acoustic signal which is applicable to a loudspeaker that generates acoustic vibration in air, a headphone that is directly held against the ear for listening, a bone conduction speaker that transfers acoustic vibration through a cranial bone to be listened by auditory nerve, and so on. Particularly, the present invention relates to a piezoelectric device for generating an acoustic signal which uses a piezoelectric element.
2. Description of Related Art
A piezoelectric device for generating an acoustic signal using a piezoelectric element typically employs a piezoelectric unimorph element or a piezoelectric bimorph element. FIGS. 1A and 1B are a perspective view and a side view, respectively, of a piezoelectric unimorph element. The piezoelectric unimorph element has a structure that a thin circular piezoelectric ceramic plate 21 having a diameter of about 20 mm and a thickness of about 0.1 to 0.3 mm is adhered to one surface of a thin circular metal plate 22 having a diameter of about 30 mm and a thickness of about 0.1 mm. FIGS. 2A and 2B are a perspective view and a side view, respectively, of a piezoelectric bimorph element. The piezoelectric bimorph element has a structure that the piezoelectric ceramic plate 21 is adhered to both surfaces of the metal plate 22.
The frequency characteristics of an acoustic pressure as an acoustic performance of the piezoelectric unimorph element and the piezoelectric bimorph element are such that a large acoustic pressure is generated within a resonant frequency of several kHz of the piezoelectric unimorph element or the piezoelectric bimorph element while the acoustic pressure decreases significantly once the frequency falls outside the resonant frequency. Therefore, those elements are mainly used in the field of a piezoelectric sounder that generates an acoustic signal with a specific frequency. Further, if the thickness of a piezoelectric ceramic plate is reduced to under 0.1 mm, for example, and the outer shape of a diaphragm is 50 mmφ, it can be used as a tweeter which is a speaker that is designed to deal with a high frequency of 1 kHz or higher.
There are various proposals to improve an acoustic pressure or frequency characteristics. FIG. 3 is a side view of a piezoelectric device for generating an acoustic signal according to a related art. The piezoelectric device for generating an acoustic signal has a structure that the center part of a piezoelectric bimorph element 23 is held by a support 25 and one end is fixed to a base 24. In this structure, a reaction force to the vibration generated at the center of the piezoelectric bimorph element 23 is transferred to the base 24 through the support 25 to thereby allow the base 24 to serve as a vibration plane. Accordingly, the area of the vibration plane is enlarged, thus enabling an increase in acoustic pressure. Further, this piezoelectric device combines the vibration mode of the piezoelectric bimorph element 23 and the vibration mode of the base 24, and has acoustic characteristics based on the combined vibration mode, thus capable of serving as a practical loudspeaker having a wide audio frequency range. Such a piezoelectric device for generating an acoustic signal is disclosed in Japanese Unexamined Patent Application Publications Nos. 2000-209697 and 2000-201398, for example.
In the use of the piezoelectric device for generating an acoustic signal for a cell phone, a portable terminal or the like, maximum miniaturization and high output are required. If a piezoelectric bimorph element is circular, its diameter is related to a resonant frequency in such a way that a smaller diameter leads to a higher resonant frequency to reduce a low frequency acoustic output. If a piezoelectric bimorph element is rectangular, its length is related to a resonant frequency in such a way that a shorter length leads to a higher resonant frequency to reduce a low frequency acoustic output. Further, its width is related to an amount of acoustic output in such a way that a narrower width leads to a smaller output. Accordingly, in order to produce a necessary acoustic frequency, it needs the displacement of mechanical vibration that is generated by a piezoelectric element in accordance therewith. Because the displacement of mechanical vibration is determined by the shape of a piezoelectric element, there is a limit to miniaturization while maintaining a desired acoustic output.
Further, a portable device such as a cell phone and a portable terminal should be resistant to dropping impact. However, in order to set the resonant frequency to a low frequency band by way of a small bend elastic coefficient, the piezoelectric unimorph element and piezoelectric bimorph element need to employ a very thin piezoelectric ceramic plate and metal plate. The mechanical strength of these elements is low and thus vulnerable to dropping impact.
Furthermore, in the use of the piezoelectric device for generating an acoustic signal using the piezoelectric unimorph element and piezoelectric bimorph element for a cell phone, a portable terminal or the like, it is typically incorporated into a case or housing. This causes vibration of a case or housing where an acoustic vibration should not be output, which leads to sound leakage to have people around a user hear the sound. It is thus unsuitable for application to the piezoelectric device for generating an acoustic signal that requires privacy feature. When the device is applied to a bone conduction speaker, the vibration occurring in a portion that is different from the portion though which vibration is transferred to a cranial bone and the airway sound are unwanted. Particularly, when a microphone is placed in the same housing, the microphone detects the leak sound to undesirably produce echo by acoustic coupling.
In order to overcome the above drawback, there has been a need for a piezoelectric device for generating an acoustic signal that is a small size, highly resistant to dropping impact, and has good acoustic performance with less sound leakage.
The present invention provides a means for generating an acoustic signal which is different from the piezoelectric unimorph element or piezoelectric bimorph element to thereby achieve size reduction, increase resistance to dropping impact, and improve acoustic performance by reducing sound leakage.